Systems and Methods for Manufacturing Waveguide Cells

ABSTRACT

Systems for the manufacturing of waveguide cells in accordance with various embodiments can be configured and implemented in many different ways. In many embodiments, various deposition mechanisms are used to deposit layer(s) of optical recording material onto a transparent substrate. A second transparent substrate can be provided, and the three layers can be laminated to form a waveguide cell. Suitable optical recording material can vary widely depending on the given application. In some embodiments, the optical recording material deposited has a similar composition throughout the layer. In a number of embodiments, the optical recording material spatially varies in composition, allowing for the formation of optical elements with varying characteristics. Regardless of the composition of the optical recording material, any method of placing or depositing the optical recording material onto a substrate can be utilized.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application claims the benefit of and priority under 35U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/663,864entitled “Method and Apparatus for Fabricating Holographic Gratings,”filed Apr. 27, 2018, U.S. Provisional Patent Application No. 62/614,813entitled “Low Haze Liquid Crystal Materials,” filed Jan. 8, 2018, U.S.Provisional Patent Application No. 62/614,831 entitled “Liquid CrystalMaterials and Formulations,” filed Jan. 8, 2018, U.S. Provisional PatentApplication No. 62/614,932 entitled “Methods for Fabricating OpticalWaveguides,” filed Jan. 8, 2018, U.S. Provisional Patent Application No.62/667,891 entitled “Method and Apparatus for Copying a Diversity ofHologram Prescriptions from a Common Master,” filed May 7, 2018, andU.S. Provisional Patent Application No. 62/703,329 entitled “Systems andMethods for Fabricating a Multilayer Optical Structure,” filed Jul. 25,2018. The disclosures of U.S. Provisional Patent Application Nos.62/663,864, 62/614,813, 62/614,831, 62/614,932, 62/667,891, and62/703,329 are hereby incorporated by reference in their entireties forall purposes.

FIELD OF THE INVENTION

The present invention generally relates to processes and apparatuses formanufacturing waveguide cells and, more specifically, manufacturingwaveguide cells utilizing deposition and printing techniques.

BACKGROUND

Waveguides can be referred to as structures with the capability ofconfining and guiding waves (i.e., restricting the spatial region inwhich waves can propagate). One class of waveguides includes opticalwaveguides, which are structures that can guide electromagnetic waves,typically those in the visible spectrum. Waveguide structures can bedesigned to control the propagation path of waves using a number ofdifferent mechanisms. For example, planar waveguides can be designed toutilize diffraction gratings to diffract and couple incident light intothe waveguide structure such that the in-coupled light can proceed totravel within the planar structure via total internal reflection(“TIR”).

Fabrication of waveguides can include the use of material systems thatallow for the recording of holographic optical elements within thewaveguides. One class of such material includes polymer dispersed liquidcrystal (“PDLC”) mixtures, which are mixtures containingphotopolymerizable monomers and liquid crystals. A further subclass ofsuch mixtures includes holographic polymer dispersed liquid crystal(“HPDLC”) mixtures. Holographic optical elements, such as volume phasegratings, can be recorded in such a liquid mixture by illuminating thematerial with two mutually coherent laser beams. During the recordingprocess, the monomers polymerize and the mixture undergoes aphotopolymerization-induced phase separation, creating regions denselypopulated by liquid crystal micro-droplets, interspersed with regions ofclear polymer. The alternating liquid crystal-rich and liquidcrystal-depleted regions form the fringe planes of the grating.

Waveguide optics, such as those described above, can be considered for arange of display and sensor applications. In many applications,waveguides containing one or more grating layers encoding multipleoptical functions can be realized using various waveguide architecturesand material systems, enabling new innovations in near-eye displays forAugmented Reality (“AR”) and Virtual Reality (“VR”), compact Heads UpDisplays (“HUDs”) for aviation and road transport, and sensors forbiometric and laser radar (“LIDAR”) applications.

SUMMARY OF THE INVENTION

One embodiment includes a method for manufacturing waveguide cells, themethod including providing a first substrate, determining a predefinedgrating characteristic, and depositing a layer of optical recordingmaterial onto the first substrate using at least one deposition head,wherein the optical recording material deposited over the grating regionis formulated to achieve the predefined grating characteristic.

In another embodiment, the method further includes providing a secondsubstrate, placing the second substrate onto the deposited layer ofoptical recording material, and laminating the first substrate, thelayer of optical recording material, and the second substrate.

In a further embodiment, depositing the layer of optical recordingmaterial includes providing a first mixture of optical recordingmaterial, providing a second mixture of optical recording material, anddepositing the first and second mixtures of optical recording materialonto the first substrate in a predetermined pattern using the at leastone deposition head.

In still another embodiment, the first mixture of optical recordingmaterial includes a first bead and the second mixture of opticalrecording material includes a second bead that is a different size fromthe first bead.

In a still further embodiment, the first mixture of optical recordingmaterial has a different percentage by weight of liquid crystals thanthe second mixture of optical recording material.

In yet another embodiment, the method further includes defining agrating region and a nongrating region on the first substrate, whereinthe first mixture of optical recording material includes a liquidcrystal and a monomer, the second mixture of optical recording materialincludes a monomer, and depositing the first and second mixtures ofoptical recording material onto the first substrate in the predeterminedpattern includes depositing the first mixture of optical recordingmaterial over the grating region and depositing the second mixture ofoptical recording material over the nongrating region.

In a yet further embodiment, the first mixture of optical recordingmaterial is a polymer dispersed liquid crystal mixture that includes amonomer, a liquid crystal, a photoinitiator dye, and a coinitiator.

In another additional embodiment, the polymer dispersed liquid crystalmixture includes an additive selected from the group that includes aphotoinitiator, nano particles, low-functionality monomers, additivesfor reducing switching voltage, additives for reducing switching time,additives for increasing refractive index modulation, and additives forreducing haze.

In a further additional embodiment, the at least one deposition headincludes at least one inkjet print head.

In another embodiment again, depositing the layer of optical recordingmaterial includes providing a first mixture of optical recordingmaterial, providing a second mixture of optical recording material,printing a first dot of the first mixture of optical recording materialusing the at least one inkjet print head, and printing a second dot ofthe second mixture of optical recording material adjacent to the firstdot using the at least one inkjet print head.

In a further embodiment again, the at least one inkjet print headincludes a first inkjet print head and a second inkjet print head anddepositing the layer of optical recording material includes providing afirst mixture of optical recording material, providing a second mixtureof optical recording material, printing the first mixture of opticalrecording material onto the first substrate using the first inkjet printhead, and printing the second mixture of optical recording material ontothe first substrate using the second inkjet print head.

In still yet another embodiment, the predefined grating characteristicincludes a characteristic selected from the group that includesrefractive index modulation, refractive index, birefringence, liquidcrystal director alignment, and grating layer thickness.

In a still yet further embodiment, the predefined grating characteristicincludes a spatial variation of a characteristic selected from the groupthat includes refractive index modulation, refractive index,birefringence, liquid crystal director alignment, and grating layerthickness.

In still another additional embodiment, the predefined gratingcharacteristic results in a grating after exposure, wherein the gratinghas a spatially varying diffraction efficiency.

A still further additional embodiment includes a system for fabricatinga grating, the system including at least one deposition head connectedto at least one reservoir containing at least one mixture of opticalrecording material, a first substrate having at least one predefinedregion for supporting gratings, a positioning element capable ofpositioning the at least one deposition head across the first substrate,wherein the at least one deposition head is configured to deposit the atleast one mixture of optical recording material onto the first substrateusing the positioning element and the deposited material provides apredefined grating characteristic within the at least one predefinedgrating region after holographic exposure.

In still another embodiment again, the at least one deposition head isconnected to a first reservoir containing a first mixture of opticalrecording material and a second reservoir containing a second mixture ofoptical recording material.

In a still further embodiment again, the first mixture of opticalrecording material includes a liquid crystal and a monomer and thesecond mixture of optical recording material includes a monomer, whereinthe at least one deposition head is configured to deposit the firstmixture of optical recording material onto the at least one predefinedgrating region.

In yet another additional embodiment, the at least one deposition headincludes at least one inkjet print head.

In a yet further additional embodiment, the predefined gratingcharacteristic includes a characteristic selected from the group thatincludes refractive index modulation, refractive index, birefringence,liquid crystal director alignment, and grating layer thickness.

In yet another embodiment again, the predefined grating characteristicresults in a grating after exposure, wherein the grating has a spatiallyvarying diffraction efficiency.

Additional embodiments and features are set forth in part in thedescription that follows, and in part will become apparent to thoseskilled in the art upon examination of the specification or may belearned by the practice of the invention. A further understanding of thenature and advantages of the present invention may be realized byreference to the remaining portions of the specification and thedrawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be more fully understood with reference to thefollowing figures and data graphs, which are presented as exemplaryembodiments of the invention and should not be construed as a completerecitation of the scope of the invention. It will apparent to thoseskilled in the art that the present invention may be practiced with someor all of the present invention as disclosed in the followingdescription.

FIG. 1A conceptually illustrates a profile view of a waveguide cell inaccordance with an embodiment of the invention.

FIG. 1B conceptually illustrates a waveguide cell with a wedge-shapedprofile in accordance with an embodiment of the invention.

FIG. 1C conceptually illustrates a top view of a waveguide cell inaccordance with an embodiment of the invention.

FIG. 2A conceptually illustrates a workcell cluster system in accordancewith an embodiment of the invention.

FIG. 2B conceptually illustrates a workcell cluster system with twodeposition workcells in accordance with an embodiment of the invention.

FIG. 3A conceptually illustrates an isometric view of a depositionworkcell in accordance with an embodiment of the invention.

FIG. 3B conceptually illustrates a top view of a deposition workcell inaccordance with an embodiment of the invention.

FIGS. 4A and 4B conceptually illustrate schematically the use of reverseray tracing to compute a compensated index modulation pattern forcoating in accordance with various embodiments of the invention.

FIGS. 5A and 5B conceptually illustrate the fundamental structuraldifferences between SBGs and SRGs.

FIG. 6 conceptually illustrates a waveguide cell with marked areas forgratings in accordance with an embodiment of the invention.

FIGS. 7A and 7B conceptually illustrate operation of a depositionmechanism utilizing a spray module in accordance with an embodiment ofthe invention.

FIGS. 8A and 8B conceptually illustrate two operational states of aspray module in accordance with an embodiment of the invention.

FIG. 9 is a flow chart conceptually illustrating a method of fabricatinga holographic grating using a selective coating process in accordancewith an embodiment of the invention.

FIG. 10 conceptually illustrates a deposition head for providingpredefined grating characteristics within grating regions in accordancewith an embodiment of the invention.

FIG. 11 conceptually illustrates operation of a deposition head fordepositing material having regions with predefined gratingcharacteristics in accordance with an embodiment of the invention.

FIG. 12 conceptually illustrates a deposition mechanism for depositingtwo grating layers in accordance with an embodiment of the invention.

FIG. 13 conceptually illustrates a system for depositing a grating layerof material and for holographically exposing the layer in accordancewith an embodiment of the invention.

FIG. 14 is a flow chart conceptually illustrating a method of depositinga film of material with regions having predefined gratingcharacteristics in accordance with an embodiment of the invention.

FIG. 15 conceptually illustrates an inkjet printing modulation scheme inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION

For the purposes of describing embodiments, some well-known features ofoptical technology known to those skilled in the art of optical designand visual displays have been omitted or simplified in order to notobscure the basic principles of the invention. Unless otherwise stated,the term “on-axis” in relation to a ray or a beam direction refers topropagation parallel to an axis normal to the surfaces of the opticalcomponents described in relation to the invention. In the followingdescription, the terms light, ray, beam and direction may be usedinterchangeably and in association with each other to indicate thedirection of propagation of light energy along rectilinear trajectories.Parts of the following description will be presented using terminologycommonly employed by those skilled in the art of optical design. Forillustrative purposes, it is to be understood that the drawings are notdrawn to scale unless stated otherwise.

Turning now to the drawings, systems and methods for manufacturingwaveguide cells are illustrated. A waveguide cell can be defined as adevice containing uncured and/or unexposed optical recording material inwhich optical elements, such as but not limited to gratings, can berecorded through exposure to certain wavelengths of electromagneticradiation. Many techniques exist for the manufacturing and constructionof waveguide cells. In many embodiments, a waveguide cell is constructedby placing a thin film of optical recording material between twotransparent substrates. In further embodiments, a workcell clustermanufacturing system is implemented to construct such waveguide cells. Aworkcell can be defined as a set of machines assigned to a particularmanufacturing task. A cluster can be defined as a group of machines thatperforms a similar function cooperatively. In some embodiments, theworkcell cluster includes a preparation workcell for preparingsubstrates for deposition, a deposition workcell for depositing anoptical recording material onto a substrate, and a lamination workcellfor laminating various layers together to form a waveguide cell.

Workcells and workcell clusters in accordance with various embodimentscan be configured and implemented in many different ways. For instance,preparation workcells can be configured to prepare substrates formaterial deposition through various processes, including but not limitedto cleaning procedures and protocols. In many embodiments, thepreparation of substrates includes glass cleaning procedures for riddingthe surfaces of the substrates of contaminants and particles. In someembodiments, procedures for increasing the surface adhesion propertiesof the substrates are implemented to further prepare the substrates formaterial deposition.

Deposition workcells can be configured to deposit one or more layers ofoptical recording material onto a transparent substrate using a varietyof different deposition and printing mechanisms. In many embodiments,additive manufacturing techniques, such as but not limited to inkjetprinting, are used to deposit the layer(s) of optical recordingmaterial. In several embodiments, spraying techniques are utilized todeposit the layer(s) of optical recording material. Suitable opticalrecording material can vary widely depending on the given application.In some embodiments, the optical recording material deposited has asimilar composition throughout the layer. In a number of embodiments,the optical recording material spatially varies in composition, allowingfor the formation of optical elements with varying characteristics.Regardless of the composition of the optical recording material, anymethod of placing or depositing the optical recording material onto asubstrate can be utilized.

Lamination workcells can be configured to laminate various layers toform a waveguide cell. In a number of embodiments, the laminationworkcell is configured to laminate and form a three-layer composite ofoptical recording material and transparent substrates. As can readily beappreciated, the number of layers and types of materials used toconstruct the waveguide cells can vary and depend on the givenapplication. For example, in some embodiments, waveguide cells can beconstructed to include protective cover layers, polarization controllayers, and/or alignment layers. In some embodiments, the system isconfigured for the production of curved waveguides and waveguide cells.Specific materials, systems, and methods for constructing waveguidecells are discussed below in further detail.

Waveguide Cells

Waveguide cells can be configured and constructed in many different waysin accordance with various embodiments of the invention. As discussedabove, in many waveguide configurations, the waveguide cell includes athin film of optical recording material sandwiched between twosubstrates. Such waveguide cells can be manufactured using variousprocesses. In many embodiments, waveguide cells can be constructed bycoating a first substrate with an optical recording material capable ofacting as an optical recording medium. Various optical recordingmaterials can be used. In some embodiments, the optical recordingmaterial is a holographic polymer dispersed liquid crystal mixture(e.g., a matrix of liquid crystal droplets). As can readily beappreciated, the choice of optical recording material and types ofmixtures utilized can depend on the given application. The opticalrecording material can be deposited using a variety of depositiontechniques. In a number of embodiments, the optical recording materialcan be deposited onto the first substrate through inkjetting, spincoating, and/or spraying processes. The deposition processes can beconfigured to deposit one or more type of optical recording material. Insome embodiments, the deposition process is configured to depositoptical recording material that spatially varies in composition across asubstrate. After deposition of the optical recording material, a secondsubstrate can be placed such that the optical recording material issandwiched between the two substrates to form a waveguide cell. Inseveral embodiments, the second substrate can be a thin protective filmcoated onto the exposed layer. In such embodiments, various techniques,including but not limited to spraying processes, can be used to coat theexposed layer with the desired film of material. In a number ofembodiments, the waveguide cell can include various additional layers,such as but not limited to polarization control layers and/or alignmentlayers. Other processes for manufacturing waveguide cells can includefilling empty waveguide cells (constructed of two substrates) with anoptical recording material using processes such as but not limited togravity filling and vacuum filling methods.

Substrates used in the construction of waveguide cells are often made oftransparent materials. In some embodiments, the substrate is an opticalplastic. In other embodiments, the substrate may be fabricated fromglass. An exemplary glass substrate is standard Corning Willow glasssubstrate (index 1.51) which is available in thicknesses down to 50micrometers. The thicknesses of the substrates can vary from applicationto application. In many embodiments, 1 mm thick glass slides are used asthe substrates. In addition to different thicknesses, substrates ofdifferent shapes, such as but not limited to rectangular and curvilinearshapes, can also be used depending on the application. Oftentimes, theshapes of the substrates can determine the overall shape of thewaveguide. In a number of embodiments, the waveguide cell contains twosubstrates that are of the same shape. In other embodiments, thesubstrates are of different shapes. As can readily be appreciated, theshapes, dimensions, and materials of the substrates can vary and dependon the specific requirements of a given application.

In many embodiments, beads or other particles are dispersed throughoutthe optical recording material to help control the thickness of thelayer of optical recording material and to help prevent the twosubstrates from collapsing onto one another. In some embodiments, thewaveguide cell is constructed with an optical recording material layersandwiched between two planar substrates. Depending on the type ofoptical recording material used, thickness control can be difficult toachieve due to the viscosity of some optical recording materials and thelack of a bounding edge for the optical recording material layer. In anumber of embodiments, the beads are relatively incompressible solids,which can allow for the construction of waveguide cells with consistentthicknesses. The size of a bead can determine a localized minimumthickness for the area around the individual bead. As such, thedimensions of the beads can be selected to help attain the desiredoptical recording material layer thickness. The beads can be made of anyof a variety of materials, including but not limited to glass andplastics. In several embodiments, the material of the beads is selectedsuch that its refractive index does not substantially affect thepropagation of light within the waveguide cell.

In some embodiments, the waveguide cell is constructed such that the twosubstrates are parallel or substantially parallel. In such embodiments,relatively similar sized beads can be dispersed throughout the opticalrecording material to help attain a uniform thickness throughout thelayer. In other embodiments, the waveguide cell has a tapered profile. Atapered waveguide cell can be constructed by dispersing beads ofdifferent sizes across the optical recording material. As discussedabove, the size of a bead can determine the local minimum thickness ofthe optical recording material layer. By dispersing the beads in apattern of increasing size across the material layer, a tapered layer ofoptical recording material can be formed when the material is sandwichedbetween two substrates.

Once constructed, waveguide cells can be used in conjunction with avariety of processes for recording optical elements within the opticalrecording material. For example, the process disclosed may incorporatedembodiments and teachings from the materials and processes, such as butnot limited to those described in U.S. patent application Ser. No.16/116,834 entitled “Systems and Methods for High-Throughput Recordingof Holographic Gratings in Waveguide Cells,” filed Aug. 29, 2018 andU.S. patent application Ser. No. 16/007,932 entitled “HolographicMaterial Systems and Waveguides Incorporating Low FunctionalityMonomers,” filed Jun. 13, 2018 The disclosures of U.S. patentapplication Ser. Nos. 16/116,834 and 16/007,932 are hereby incorporatedin their entireties for all purpose.

A profile view of a waveguide cell 100 in accordance with an embodimentof the invention is conceptually illustrated in FIG. 1A. As shown, thewaveguide cell 100 includes a layer of optical recording material 102that can be used as a recording medium for optical elements, such as butnot limited to gratings. The optical recording material 102 can be anyof a variety of compounds, mixtures, or solutions, such as but notlimited to the HPDLC mixtures described in the sections above. In theillustrative embodiment, the optical recording material 102 is sandwichbetween two parallel glass plates 104, 106. The substrates can bearranged in both parallel and non-parallel configurations. FIG. 1Bconceptually illustrates a profile view of a tapered waveguide cell 108utilizing beads 110, 112, and 114 in accordance with an embodiment ofthe invention. As shown, beads 110, 112, and 114 vary in size and aredispersed throughout an optical recording material 116 sandwiched by twoglass plates 118, 120. During construction of the waveguide cell, thelocal thickness of an area of the optical recording material layer islimited by the sizes of the beads in that particular area. By dispersingthe beads in an increasing order of sizes across the optical recordingmaterial, a tapered waveguide cell can be constructed when thesubstrates are placed in contact with the beads. As discussed above,substrates utilized in waveguide cells can vary in thicknesses andshapes. In many embodiments, the substrate is rectangular in shape. Insome embodiments, the shape of the waveguide cell is a combination ofcurvilinear components. FIG. 1C conceptually illustrates a top view of awaveguide cell 122 having a curvilinear shape in accordance with anembodiment of the invention.

Although FIGS. 1A-1C illustrate specific waveguide cell constructionsand arrangements, waveguide cells can be constructed in many differentconfigurations and can use a variety of different materials depending onthe specific requirements of a given application. For example,substrates can be made of transparent plastic polymers instead of glass.Additionally, the shapes and sizes of the waveguide cells can varygreatly and can be determined by various factors, such as but notlimited to the application of the waveguide, ergonomic considerations,and economical factors. In many embodiments, the substrates are curved,allowing for the production of waveguides with curved cross sections.

Grating Structures

Waveguide cells in accordance with various embodiments of the inventioncan incorporate a variety of light-sensitive materials. In manyembodiments, the waveguide cell incorporates a holographic polymerdispersed liquid crystal mixture that functions as an optical recordingmedium in which optical elements can be recorded. Optical elements caninclude many different types of gratings capable of exhibiting differentoptical properties. One type of grating that can be recorded inwaveguide cells is a volume Bragg grating, which can be characterized asa transparent medium with a periodic variation in its refractive index.This variation can allow for the diffraction of incident light ofcertain wavelengths at certain angles. Volume Bragg gratings can havehigh efficiency with little light being diffracted into higher orders.The relative amount of light in the diffracted and zero order can bevaried by controlling the refractive index modulation of the grating.

One class of gratings used in holographic waveguide devices is theSwitchable Bragg Grating (“SBG”). An SBG is a diffractive device thatcan be formed by recording a volume phase grating in an HPDLC mixture(although other materials can be used). SBGs can be fabricated by firstplacing a thin film of a mixture of photopolymerizable monomers andliquid crystal material between glass plates or substrates, which formsa waveguide cell. One or both glass plates can support electrodes,typically transparent tin oxide films, for applying an electric fieldacross the film. SBGs can be implemented as waveguide devices in whichthe HPDLC forms either the waveguide core or an evanescently coupledlayer in proximity to the waveguide. The glass plates used to form theHPDLC cell can provide a total internal reflection light guidingstructure. Light is coupled out of the SBG when the switchable gratingdiffracts the light at an angle beyond the TIR condition.

The grating structure in an SBG can be recorded in the film of HPDLCmaterial through photopolymerization-induced phase separation usinginterferential exposure with a spatially periodic intensity modulation.Factors such as but not limited to control of the irradiation intensity,component volume fractions of the HPDLC material, and exposuretemperature can determine the resulting grating morphology andperformance. During the recording process, the monomers polymerize andthe mixture undergoes a phase separation. The LC molecules aggregate toform discrete or coalesced droplets that are periodically distributed inpolymer networks on the scale of optical wavelengths. The alternatingliquid crystal-rich and liquid crystal-depleted regions form the fringeplanes of the grating, which can produce Bragg diffraction with a strongoptical polarization resulting from the orientation ordering of the LCmolecules in the droplets. The resulting volume phase grating canexhibit very high diffraction efficiency, which may be controlled by themagnitude of the electric field applied across the HPDLC layer. When anelectric field is applied to the hologram via transparent electrodes,the natural orientation of the LC droplets is changed, causing therefractive index modulation of the fringes to reduce and the hologramdiffraction efficiency to drop to very low levels. The diffractionefficiency of the device can be adjusted, by means of the appliedvoltage, over a continuous range from near 100% efficiency with novoltage applied to essentially zero efficiency with a sufficiently highvoltage applied. In certain types of HPDLC devices, phase separation ofthe LC material from the polymer can be accomplished to such a degreethat no discernible droplet structure results. An SBG can also be usedas a passive grating. In this mode, its chief benefit is a uniquely highrefractive index modulation. SBGs can be used to provide transmission orreflection gratings for free space applications. SBGs can be implementedas waveguide devices in which the HPDLC forms either the waveguide coreor an evanescently coupled layer in proximity to the waveguide. Theglass plates used to form the HPDLC cell provide a total internalreflection light guiding structure. Light can be coupled out of the SBGwhen the switchable grating diffracts the light at an angle beyond theTIR condition.

In many embodiments, SBGs are recorded in a uniform modulation material,such as POLICRYPS or POLIPHEM having a matrix of solid liquid crystalsdispersed in a liquid polymer. Exemplary uniform modulation liquidcrystal-polymer material systems are disclosed in United State PatentApplication Publication No.: US2007/0019152 by Caputo et al and PCTApplication No.: PCT/EP2005/006950 by Stumpe et al. both of which areincorporated herein by reference in their entireties. Uniform modulationgratings are characterized by high refractive index modulation (andhence high diffraction efficiency) and low scatter. In some embodiments,at least one of the gratings is recorded a reverse mode HPDLC material.Reverse mode HPDLC differs from conventional HPDLC in that the gratingis passive when no electric field is applied and becomes diffractive inthe presence of an electric field. The reverse mode HPDLC may be basedon any of the recipes and processes disclosed in PCT Application No.:PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMERDISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES. Optical recordingmaterial systems are discussed below in further detail.

Optical Recording Material Systems

HPDLC mixtures in accordance with various embodiments of the inventiongenerally include LC, monomers, photoinitiator dyes, and coinitiators.The mixture (often referred to as syrup) frequently also includes asurfactant. For the purposes of describing the invention, a surfactantis defined as any chemical agent that lowers the surface tension of thetotal liquid mixture. The use of surfactants in PDLC mixtures is knownand dates back to the earliest investigations of PDLCs. For example, apaper by R. L Sutherland et al., SPIE Vol. 2689, 158-169, 1996, thedisclosure of which is incorporated herein by reference, describes aPDLC mixture including a monomer, photoinitiator, coinitiator, chainextender, and LCs to which a surfactant can be added. Surfactants arealso mentioned in a paper by Natarajan et al, Journal of NonlinearOptical Physics and Materials, Vol. 5 No. I 89-98, 1996, the disclosureof which is incorporated herein by reference. Furthermore, U.S. Pat. No.7,018,563 by Sutherland; et al., discusses polymer-dispersed liquidcrystal material for forming a polymer-dispersed liquid crystal opticalelement including: at least one acrylic acid monomer; at least one typeof liquid crystal material; a photoinitiator dye; a coinitiator; and asurfactant. The disclosure of U.S. Pat. No. 7,018,563 is herebyincorporated by reference in its entirety.

The patent and scientific literature contains many examples of materialsystems and processes that can be used to fabricate waveguidesincorporating volume gratings, including investigations into formulatingsuch material systems for achieving high diffraction efficiency, fastresponse time, low drive voltage, and so forth. U.S. Pat. No. 5,942,157by Sutherland, and U.S. Pat. No. 5,751,452 by Tanaka et al. bothdescribe monomer and liquid crystal material combinations suitable forfabricating waveguides incorporating volume gratings. Examples ofrecipes can also be found in papers dating back to the early 1990s, manyof which disclose the use of acrylate monomers, including:

-   -   R. L. Sutherland et al., Chem. Mater. 5, 1533 (1993), the        disclosure of which is incorporated herein by reference,        describes the use of acrylate polymers and surfactants.        Specifically, the recipe includes a crosslinking multifunctional        acrylate monomer; a chain extender N-vinyl pyrrolidinone, LC E7,        photo-initiator rose Bengal, and coinitiator N-phenyl glycine.        Surfactant octanoic acid was added in certain variants.    -   Fontecchio et al., SID 00 Digest 774-776, 2000, the disclosure        of which is incorporated herein by reference, describes a UV        curable HPDLC for reflective display applications including a        multi-functional acrylate monomer, LC, a photoinitiator, a        coinitiators, and a chain terminator.    -   Y. H. Cho, et al., Polymer International, 48, 1085-1090, 1999,        the disclosure of which is incorporated herein by reference,        discloses HPDLC recipes including acrylates.    -   Karasawa et al., Japanese Journal of Applied Physics, Vol. 36,        6388-6392, 1997, the disclosure of which is incorporated herein        by reference, describes acrylates of various functional orders.    -   T. J. Bunning et al., Polymer Science: Part B: Polymer Physics,        Vol. 35, 2825-2833, 1997, the disclosure of which is        incorporated herein by reference, also describes multifunctional        acrylate monomers.    -   G. S. Iannacchione et al., Europhysics Letters Vol. 36 (6).        425-430, 1996, the disclosure of which is incorporated herein by        reference, describes a PDLC mixture including a penta-acrylate        monomer, LC, chain extender, coinitiators, and photoinitiator.

Acrylates offer the benefits of fast kinetics, good mixing with othermaterials, and compatibility with film forming processes. Sinceacrylates are cross-linked, they tend to be mechanically robust andflexible. For example, urethane acrylates of functionality 2 (di) and 3(tri) have been used extensively for HPDLC technology. Higherfunctionality materials such as penta and hex functional stems have alsobeen used.

Although HPDLC mixtures with specific components are discussed above inrelation with their suitable uses as the optical recording material in awaveguide cell, specific formulations of optical recording materials canvary widely and can depend on the specific requirements of a givenapplication. Such considerations can include diffraction efficiency(“DE”), haze, solar immunity, transparency, and switching requirements.

Embodiments of S & P Polarized RMLCM Materials

The S and P polarization response of a grating containing LC can dependon the average LC director orientations relative to the gratingK-vector. Typically, the directors are substantially parallel to theK-vector, giving a strong P-response and a weaker S-response. If the LCdirectors are not aligned, the grating can have a strong S-response.Many embodiments of the invention include reactive monomer liquidcrystal mixture (“RMLCM”) material systems configured to incorporate amixture of LCs and monomers (and other components including:photoinitiator dye, coinitiators, surfactant), which under holographicexposure undergo phase separation to provide a grating in which at leastone of the LCs and at least one of the monomers form a first HPDLCmorphology that provides a P polarization response and at least one ofthe LCs and at least one of the monomers form a second HPDLC morphologythat provides a S polarization response. In various such embodiments,the material systems include an RMLCM, which includes photopolymerizablemonomers composed of suitable functional groups (e.g., acrylates,mercapto-, and other esters, among others), a cross-linking agent, aphoto-initiator, a surfactant and a liquid crystal.

Turning to the components of the material formulation, any encapsulatingpolymer formed from any single photo-reactive monomer material ormixture of photo-reactive monomer materials having refractive indicesfrom about 1.5 to 1.9 that crosslink and phase separate when combinedcan be utilized. Exemplary monomer functional groups usable in materialformulations according to embodiments include, but are not limited to,acrylates, thiol-ene, thiol-ester, fluoromonomers, mercaptos,siloxane-based materials, and other esters, etc. Polymer cross-linkingcan be achieved through different reaction types, including but notlimited to optically-induced photo-polymerization, thermally-inducedpolymerization, and chemically-induced polymerization.

These photopolymerizable materials can be combined in a biphase blendwith a second liquid crystal material. Any suitable liquid crystalmaterial having ordinary and extraordinary refractive indices matched tothe polymer refractive index can be used as a dopant to balance therefractive index of the final RMLCM material. The liquid crystalmaterial can be manufactured, refined, or naturally occurring. Theliquid crystal material includes all known phases of liquidcrystallinity, including the nematic and smectic phases, the cholestericphase, the lyotropic discotic phase. The liquid crystal can exhibitferroelectric or antiferroelectric properties and/or behavior.

Any suitable photoinitiator, co-initiator, chain extender and surfactant(such as for example octanoic acid) suitable for use with the monomerand LC materials can be used in the RMLCM material formulation. It willbe understood that the photo-initiator can operate in any desiredspectral band including the in the UV and/or in the visible band.

In various embodiments, the LCs can interact to form an LC mixture inwhich molecules of two or more different LCs interact to form anon-axial structure which interacts with both S and P polarizations. Thewaveguide can also contain an LC alignment material for optimizing theLC alignment for optimum S and P performance. In many embodiments, theratio of the diffraction efficiencies of the P- and S-polarized light inthe PDLC morphology is maintained at a relative ratio of from 1.1:1 to2:1, and in some embodiments at around 1.5:1. In other embodiments, themeasured diffraction efficiency of P-polarized light is from greaterthan 20% to less than 60%, and the diffraction efficiency forS-polarized light is from greater than 10% to less than 50%, and in someembodiments the diffraction efficiency of the PDLC morphology forP-polarization is around 30% and the diffraction efficiency of the PDLCmorphology for S-polarization is around 20%. This can be compared withconventional PDLC morphologies where the diffraction efficiency forP-polarization is around 60% and for S-polarization is around 1 (i.e.,the conventional P-polarization materials have very low or negligibleS-components).

Mixtures Incorporating Nanoparticles

In many embodiments, the reactive monomer liquid crystal mixture canfurther include chemically active nanoparticles disposed within the LCdomains. In some such embodiments, the nanoparticles are carbon nanotube(“CNT”) or nanoclay nanoparticle materials within the LC domains.Embodiments are also directed to methods for controlling the nanoclayparticle size, shape, and uniformity. Methods for blending anddispersing the nanoclay particles can determine the resulting electricaland optical properties of the device. The use of nanoclays in HPDLC isdiscussed in PCT Application No.: PCT/GB2012/000680, entitledIMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALSAND DEVICES.

The nanoclay nanoparticles can be formed from any naturally occurring ormanufactured composition, as long as they can be dispersed in the liquidcrystal material. The specific nanoclay material to be selected dependsupon the specific application of the film and/or device. Theconcentration and method of dispersion also depends on the specificapplication of the film and/or device. In many embodiments, the liquidcrystal material is selected to match the liquid crystal ordinary indexof refraction with the nanoclay material. The resulting compositematerial can have a forced alignment of the liquid crystal molecules dueto the nanoclay particle dispersion, and the optical quality of the filmand/or device can be unaffected. The composite mixture, which includesthe liquid crystal and nanoclay particles, can be mixed to an isotropicstate by ultrasonication. The mixture can then be combined with anoptically crosslinkable monomer, such as acrylated or urethane resinthat has been photoinitiated, and sandwiched between substrates to forma cell (or alternatively applied to a substrate using a coatingprocess).

In various embodiments, nanoparticles are composed of nanoclaynanoparticles, preferably spheres or platelets, with particle size onthe order of 2-10 nanometers in the shortest dimension and on the orderof 10 nanometers in the longest dimension. Desirably, the liquid crystalmaterial is selected to match the liquid crystal ordinary index ofrefraction with the nanoclay material. Alternatively, the nanoparticlescan be composed of material having ferroelectric properties, causing theparticles to induce a ferroelectric alignment effect on the liquidcrystal molecules, thereby enhancing the electro-optic switchingproperties of the device. In another embodiment of the invention, thenanoparticles are composed of material having ferromagnetic properties,causing the particles to induce a ferromagnetic alignment effect on theliquid crystal molecules, thereby enhancing the electro-optic switchingproperties of the device. In another embodiment of the invention, thenanoparticles have an induced electric or magnetic field, causing theparticles to induce an alignment effect on the liquid crystal molecules,thereby enhancing the electro-optic switching properties of the device.Exemplary nanoparticles used in other contexts including thermoplastics,polymer binders, etc. are disclosed in U.S. Pat. Nos. 7,068,898;7,046,439; 6,323,989; 5,847,787; and U.S. Patent Pub. Nos. 2003/0175004;2004/0156008; 2004/0225025; 2005/0218377; and 2006/0142455, thedisclosures of which are incorporated herein by reference.

The nanoclay can be used with its naturally occurring surfaceproperties, or the surface can be chemically treated for specificbinding, electrical, magnetic, or optical properties. Preferably, thenanoclay particles will be intercalated, so that they disperse uniformlyin the liquid crystalline material. The generic term “nanoclay” as usedin the discussion of the present invention can refer to naturallyoccurring montmorillonite nanoclay, intercalated montmorillonitenanoclay, surface modified montmorillonite nanoclay, and surface treatedmontmorillonite nanoclay. The nanoparticles can be useable ascommercially purchased, or they may need to be reduced in size oraltered in morphology. The processes that can be used include chemicalparticle size reduction, particle growth, grinding of wet or dryparticles, milling of large particles or stock, vibrational milling oflarge particles or stock, ball milling of particles or stock,centrifugal ball milling of particles or stock, and vibrational ballmilling of particles or stock. All of these techniques can be performedeither dry or with a liquid suspension. The liquid suspension can be abuffer, a solvent, an inert liquid, or a liquid crystal material. Oneexemplary ball milling process provided by Spex LLC (Metuchen, N.J.) isknown as the Spex 8000 High Energy Ball Mill. Another exemplary process,provided by Retsch (France), uses a planetary ball mill to reducemicrometer size particles to nanoscale particles.

The nanoparticles can be dispersed in the liquid crystal material priorto polymer dispersion. Dry or solvent suspended nanoparticles can beultrasonically mixed with the liquid crystal material or monomers priorto polymer dispersion to achieve an isotropic dispersion. Wet particlesmay need to be prepared for dispersion in liquid crystal, depending onthe specific materials used. If the particles are in a solvent or liquidbuffer, the solution can be dried, and the dry particles dispersed inthe liquid crystal as described above. Drying methods includeevaporation in air, vacuum evaporation, purging with inert gas likenitrogen and heating the solution. If the particles are dispersed in asolvent or liquid buffer with a vapor pressure lower than the liquidcrystal material, the solution can be mixed directly with the liquidcrystal, and the solvent can be evaporated using one of the abovemethods leaving behind the liquid crystal/nanoparticle dispersion. Inone embodiment of the invention, the optical film includes a liquidcrystal material and a nanoclay nanoparticle, where a nanoparticle is aparticle of material with size less than one micrometer in at least onedimension. The film can be isotropically distributed.

Although nanoclay materials are discussed, in many embodiments CNT isused as an alternative to nanoclay as a means for reducing voltage. Theproperties of CNT in relation to PDLC devices are reviewed by E. H. Kimet. al. in Polym. Int. 2010; 59: 1289-1295, the disclosure of which isincorporated herein by reference in its entirety. PDLC films have beenfabricated with varying amounts of multi-walled carbon nanotubes(“MWCNTs”) to optimize the electro-optical performance of the PDLCfilms. The MWCNTs were well dispersed in the prepolymer mixture up to0.5 wt %, implying that polyurethane acrylate (“PUA”) oligomer chainswrap the MWCNTs along their length, resulting in high diffractionefficiency and good phase separation. The hardness and elastic modulusof the polymer matrix were enhanced with increasing amounts of MWCNTsbecause of the reinforcement effect of the MWCNTs with intrinsicallygood mechanical properties. The increased elasticity of the PUA matrixand the immiscibility between the matrix and the liquid crystalsgradually increased the diffraction efficiency of the PDLC films.However, the diffraction efficiency of PDLC films with more than 0.05 wt% MWCNTs was reduced, caused by poor phase separation between the matrixand LCs because of the high viscosity of the reactive mixture. PDLCfilms showing a low driving voltage (75%) could be obtained with 0.05 wt% MWCNTs at 40 wt % LCs.

In embodiments where the PDLC materials incorporate such nanoparticles,reductions of switching voltage and improvements to the electro-opticproperties of a polymer dispersed liquid crystal film and/or polymerdispersed liquid crystal device can be obtained by includingnanoparticles in the liquid crystal domains. The inclusion ofnanoparticles serves to align the liquid crystal molecules and to alterthe birefringent properties of the film through index of refractionaveraging. In addition, the inclusion of the nanoparticles improves theswitching response of the liquid crystal domains.

Monomer Functionality

RMLCM material systems in accordance with various embodiments can beformulated in a variety of ways. In many embodiments, the materialsystem is an RMLCM that includes at least one LC, at least onemulti-functional monomer, a photo-initiator, a dye, and at least onemono-functional monomer. Along with several factors, such as but notlimited to recording beam power/wavelength, grating periodicity, andgrating thickness, the specific mixture of components and their percentcomposition can determine the diffraction efficiency of the resultingHPDLC gratings. Inhomogeneous polymerization due to the spatiallyperiodic irradiation intensity of the exposure can be the driving forceto segregate monomers and LCs and to order the orientation of LCmolecules, which can influence the diffraction efficiencies of the HPDLCgratings. Oftentimes, the diffusion coefficient of monomers depends ontheir molecular weight and reactivity. It has been shown that a varietyof monomer molecular weights or functional numbers can yield a complexdistribution of polymer and LC phases. In many cases, molecularfunctionality can be critical in achieving efficient phase separationand the formation of gratings with high diffraction efficiency. As such,many embodiments of the invention include material systems formulatedwith specific mixes of monomers that are chosen, at least in part, fortheir functionality so as to influence the diffraction efficiency andindex modulation of the resulting grating structure. Otherconsiderations in formulating such a mixture can include but are notlimited to the properties of the recording beam and the thickness of thegratings. For the purposes of describing this invention, thefunctionality of a monomer refers to the number of reactive sites oneach monomer unit.

The effects of varying monomer functionality in HPDLC material systemshave been studied to some degree in the scientific literature. Suchstudies have generally examined the effects of the effective, oraverage, functionality of a mixture with regards to grating formationand performance. For example, in a paper by Pogue et al., Polymer 41(2000) 733-741, the disclosure of which is incorporated herein byreference, investigations were conducted in floodlit PDLCs andholographic PDLC gratings to show that a decrease in effective monomerfunctionality general leads to decreased LC phase separation.

Many embodiments in accordance with the invention include investigationsinto mixtures with specific blends of monomers of low functionality thatcan result in the formation of gratings having high diffractionefficiency and efficient phase separation. While the scientificliterature typically emphasizes the use of high functionality monomers,various embodiments in accordance with the invention are focused on theuse of monomers of low functionality in certain applications. In someembodiments, the monomers within the mixture are either mono-functionalmonomers or bi-functional monomers. In a number of embodiments,tri-functional monomers are also included. In such mixtures, thetri-functional monomers are typically included at a low concentration,such as lower than 5 wt %.

Mixtures including low functional monomers can behave differentlydepending on a variety of factors, such as but not limited to thewavelength sensitivity of the material system, thickness of the HPDLC tobe formed, and exposure temperature. In the scientific literature,investigations into PDLC material systems typically include UV sensitivematerial systems since material reaction efficiency in general istypically poor with visible light systems. However, formulations inaccordance with various embodiments of the invention have been able toreach high diffraction efficiency (>80%) with low haze using lowfunctionality monomers that are sensitive (polymerizes) to visiblelight. In further embodiments, the material systems include monomersthat are sensitive to green light, such as light with wavelengthsranging from 495-570 nm. In addition to different light systems,performance of the HPDLC mixtures can depend on the thickness of thewaveguide cell in which gratings are formed. For example, for a givenmaterial system, different thicknesses of deposited films can formwaveguides with different amounts of haze. Although grating thicknesseshave been explored in the patent and scientific literature, suchinvestigations are focused on relatively thick gratings. In a number ofembodiments, the material system is formulated for use in waveguideswith thin form factors. In further embodiments, the material system isformulated for use in manufacturing waveguides having HPDLC layers withthicknesses of less than 10 μm. and gratings with more than 80%diffraction efficiency. In further embodiments, the material system isformulated for use in a waveguide having a 2-3 μm thick HPDLC layer andgratings with 80-90% diffraction efficiency. The material system canalso be formulated for manufacturing such waveguides with low haze. Inseveral embodiments, the material system can form HPDLC layers havingless than 1% haze. Waveguide haze is the integrated effect of lightinteracting with material and surface inhomogeneities over many beambounces. The impact on the ANSI contrast, the ratio of averaged white toblack measurements taken from a checkerboard pattern, can be dramaticowing to the scatter contribution to the black level. Haze is mostly dueto wide-angle scatter by LC droplets and other small particles orscattering centers resulting from incomplete phase separation of theLC/monomer mixture during grating recording. Haze can also arise, atleast partly, from narrow angle scatter generated by large-scalenonuniformities, leading to a loss of see-through quality and reducedimage sharpness. Some waveguide applications such as aircraft HUDs,which use 1-D beam expansion in thick waveguides, produce as few as 7bounces, allowing up to 80:1 contrast. However, in thin waveguides ofthe type use in near eye displays the number of bounces may increase bya factor of 10 making the need for haze control more acute.

RMLCM recipes can be optimized for specific thicknesses of HPDLC layers.In many embodiments, the RMLCM recipe is optimized for a ˜3 μm thickuniform modulation gratings designed to have a refractive indexmodulation of ˜0.16. As can readily be appreciated, the specificthickness of the waveguide parts to be fabricated can vary and candepend on the specific requirements of a given application. In a numberof embodiments, the waveguide parts can be fabricated with 90%transmission and 0.3% haze. In other embodiments, the waveguide partscan be fabricated with ˜0.1% haze (with ˜0.01% haze recorded inunexposed samples of the same material). In some embodiments, the RMLCMcan be formulated for fabricating waveguide parts containing haze ofless than 0.05%.

Transmission haze can be defined as the percentage of light thatdeviates from desired beam direction by more the 2.5 degrees on average(according to the ASTM D1003 standard). The clarity of a waveguide canbe characterized by the amount of narrow angle scattered light (at anangle less than 2.5° from the normal to the waveguide surface).Transmission can be defined as the amount of light transmitted throughthe waveguide without being scattered. To assess general material haze,the scatter can be measured around a vector normal to a waveguide TIRsurface. To assess holographic haze, the scatter can be measured aroundprincipal diffraction directions (passing through the center of the eyebox). The procedures for measurement of haze, clarity and transmissionare defined in the ASTM D1003 International test standards, in which“Procedure A” uses a haze meter and “Procedure B” uses aspectrophotometer. An exemplary instrument for measuring haze is theBYK-Gardner HAZE Guard II equipment.

In many embodiments, the RMLCM mixture includes a liquid crystalmixture, a complex mixture of acrylates and acrylate esters, Dynasylan®MEMO, and photoinitiators. In further embodiments, the RMLCM includesEHA and DFHA. Depending on the specific mix of components and theirpercent composition, the resulting grating can have vastly differentcharacteristics. In some embodiments, the proportion of LC by weight isgreater than 30%. In further embodiments, the proportion of LC isgreater than 35 wt %. In some embodiments, the mixture includes liquidcrystal with high birefringence. In further embodiments, the highbirefringence liquid crystal accounts for more than 20 wt % of themixture. In a number of embodiments, dye and photo-initiators accountfor less than 5 wt % of the mixture.

Nematic LC materials can provide a range of birefringence (which cantranslate to refractive index modulation). Low to medium birefringencetypically covers the range of 0.09-0.12. However, gratings can bedesigned using much lower birefringence values, including gratings inwhich the birefringence varies along the grating. Such gratings can beused to extract light from waveguides with low efficiency at one end ofthe grating and high efficiency at the other end of the grating toprovide spatially uniform output illumination. High birefringence(nematic LC) is typically the range of 0.2-0.5. Even higher values arepossible. Nematic liquid crystals, compounds, and mixtures with positivedielectric anisotropies (i.e., LCs for which the dielectric constant isgreater in the long molecular axis than that in the other directions)are review in a paper by R. Dabrowski et al., “High Birefringence LiquidCrystals”; Crystals; 2013; 3; 443-482, the disclosure of which isincorporated herein by reference.

The functionality of the monomers in the mixtures can greatly influencethe diffraction efficiency of the resulting grating. In manyembodiments, the mixture includes at least one mono-functional monomerand at least one multifunctional monomer in varying concentrations. Inseveral embodiments, the concentration of mono-functional monomer withinthe mixture ranges from 1-50 wt %. The monofunctional monomer caninclude aliphatic/aromatic groups and an adhesion promoter. In someembodiments, the proportion of multi-functional monomers present in themixture is in the range of 2-30 wt %. Multi-functional monomers inaccordance with various embodiments of the invention typically includemonomers of low functionality. In a number of embodiments, the mixtureincludes a bi-functional monomer at a low concentration. In furtherembodiments, the mixture includes bi-functional monomers at less than 15wt %. Depending on the type and concentration of bi-functional monomerin the mixture, adequate phase separation and grating formation canoccur. In the illustrative embodiment, the mono-functional monomer,bi-functional monomer and LC have relative weight ratios of 30%, 14%,and 40%, which resulted in a formulation that allowed for the recordingof gratings with a diffraction efficiency higher than 90% and an indexmodulation of around 0.12.

As can readily be appreciated, percent composition of each componentwithin an RMLCM can vary widely. Formulations of such material systemscan be designed to achieve certain characteristics in the resultinggratings. In many cases, the RMLCM is formulated to have as high adiffraction efficiency as possible.

Workcell Cluster for Manufacturing Waveguide Cells

Waveguide cell manufacturing systems in accordance with variousembodiments of the invention can be implemented as workcell clusters. Bycompartmentalizing different manufacturing steps into workcells, modularsystems can be implemented. In many embodiments, a workcell clusterincludes a preparation workcell for preparing substrates for materialdeposition, a deposition workcell for depositing an optical recordingmaterial onto a substrate, and a lamination workcell for laminatingvarious layers together to construct a waveguide cell. Workcells can beconfigured in various ways to implement different manufacturingprocesses for waveguide cells. In some embodiments, the workcells arelinked and configured such that the output of one workcell istransferred to another workcell, forming a manufacturing assembly line.The transferring mechanism can be implemented in a variety of ways, suchas but not limited to the use of mechanical arms, suction, and/or aconveyor system. In several embodiments, the products are manuallytransferred. FIG. 2A conceptually illustrates a workcell cluster system200 in accordance with an embodiment of the invention. In theillustrative embodiment, the system 200 includes a preparation workcell202, a deposition workcell 204, and a lamination workcell 206. As shown,arrows 208 indicate a sequential workflow relationship among theworkcells.

One advantage in a modular system is the ability to vary the number ofworkcells dedicated to a particular task to improve throughput byoptimizing workcell use and reducing workcell downtime. For example, awaveguide cell manufactured with different optical recording materialsmay result in different deposition times. In such embodiments, thenumber of deposition workcells can vary accordingly to balance out thetask completion time of each workcell such as to minimize the overalldowntime of the workcells. FIG. 2B conceptually illustrates a workcellcluster system 210 with two deposition workcells 212, 214 in accordancewith an embodiment of the invention. In the illustrative embodiment, thesystem 210 includes a preparation workcell 216, two deposition workcells212, 214, and a lamination workcell 218. Dotted arrows 220 indicate thatoutput from the preparation workcell 216 can be received by eitherdeposition workcell 212, 214. Such a system can be ideally implementedwhen the completion time for a single deposition process isapproximately twice as long as the completion time for other processes.

Although FIGS. 2A and 2B conceptually illustrate specific workcellcluster system configurations, workcell clusters in accordance withvarious embodiments of the invention can be configured in numerous waysdepending on the specific requirements of the given application. Forexample, workcell clusters can be configured to have different workflowpaths, types of workcells, and/or numbers of workcells.

Due to the sensitive nature of some materials and processes associatedwith waveguide cell fabrication, workcells can be configured to provideprotection from environmental light and contaminants. In manyembodiments, optical filters cover the workcell in order to reduceand/or prevent unwanted light from interacting with the opticalrecording material, which is typically a photosensitive material.Depending on the specific type of optical recording material, thedeposition workcell can be lined with an appropriate optical filter thatprevent light of certain wavelengths from entering the workcell andexposing the optical recording material. In addition to thereduction/prevention of light contamination, workcells can also beconfigured to reduce particulate contamination. In several embodiments,the workcell is configured to operate in an environment with minimal aircontamination. A low-particulate environment can be achieved in manydifferent ways, including but not limited to the use of air filters. Ina number of embodiments, air filters employing laminar airflowprinciples are implemented. Contamination reduction/prevention systemssuch as those described above can be implemented separately or incombination. Although specific systems are described, workcells inaccordance with various embodiments of the invention can be constructedin various ways as to alter the working environment in a desired manner.For example, in several embodiments, the workcell is configured tooperate in a vacuum. Specific workcells and their implementations andconstructions are described in the sections below in further detail.

Preparation Workcell

Waveguide cells in accordance with various embodiments of the inventionare typically composed of a layer of optical recording materialsandwiched between two substrates. Manufacturing techniques forconstructing such waveguide cells in accordance with various embodimentsof the invention can include a deposition step where a layer of opticalrecording material is deposited onto one of the substrate. In manyembodiments, a preparation workcell can be implemented to perform acleaning/preparation procedure on the substrates to prepare them for thedeposition step. Preparing substrates, such as but not limited to glassplates, can include ridding the surfaces of contaminants and increasingthe surface adhesion properties for better material deposition.

Preparation workcells can be configured to implement various cleaningand preparation protocols. Mechanical arms and/or suction apparatusescan be used to maneuver the substrates throughout the workcell. In manyembodiments, the preparation workcells are configured to clean glasssubstrates using various solvents and solutions, including but notlimited to soap solutions, acid washes, acetone, and various types ofalcohols. In some embodiments, several types of solvents and/orsolutions are used in conjunction. For example, in several embodiments,methanol or isopropanol can be administered after acetone to rinse offexcess acetone. In a number of embodiments, deionized water is used torinse off excess solvents or solutions. The solvents can be administeredin several ways, including but not limited to the use of nozzles andbaths. After cleaning, the workcell can be configured to dry thesubstrates using an inert gas, such as nitrogen, and/or a heatingelement.

In many embodiments, the cleaning process includes a sonication step. Inseveral embodiments, the substrate is placed in a chamber containing asolution and a transducer is used to produce ultrasonic waves. Theultrasonic waves can agitate the solution and remove contaminantsadhered to the substrates. The treatment can vary in duration dependingon several factors and can be performed with different types ofsubstrates. Deionized water or cleaning solutions/solvents can be useddepending on the type of contamination and the type of substrate.

In many embodiments, the preparation workcell is configured to implementa plasma chamber to plasma treat the surfaces of the substrates. In someembodiments, the substrates are made of glass. Existing in the form ofions and electrons, plasma is essentially an ionized gas that has beenelectrified with extra electrons in both negative and positive states.Plasma can be used to treat the surface of the substrate to removecontaminants and/or prepare the surface for material deposition byincreasing the surface energy to improve adhesion properties. In anumber of embodiments, the workcell includes a vacuum pump, which can beused to create a vacuum under which the plasma treatment can beperformed.

As can readily be appreciated, preparation workcells in accordance withvarious embodiments of the invention can be configured to performcombinations of various steps to implement a specific cleaning protocolaccording to the requirements of a given application. Although specificpreparation workcells for preparing glass plates are discussed above,preparation workcells can be implemented to preform various preparatorysteps for a variety of different substrates, including but not limitedto plastics.

Deposition Workcell

Waveguide cell manufacturing systems can utilize various techniques forplacing optical recording materials in between two substrates.Manufacturing systems in accordance with various embodiments of theinvention can utilize a deposition process where a film of opticalrecording material is deposited onto a substrate, and the composite islaminated along with a second substrate to form a three-layer laminate.In many embodiments, the manufacturing system is a workcell cluster thatincludes a deposition workcell for depositing a film of opticalrecording material onto a substrate. Such deposition workcells can beconfigured to receive substrates from preparation workcells. In someembodiments, the deposition workcell includes a stage for supporting thesubstrate and at least one deposition mechanism for depositing materialonto the substrate. Any of a variety of deposition heads can beimplemented to perform as the deposition mechanism. In severalembodiments, spraying mechanisms such as but not limited to sprayingnozzles are implemented to deposit optical recording material onto asubstrate. In some embodiments, the optical recording material isdeposited using a printing mechanism. Depending on the type ofdeposition mechanism/head implemented, several different depositioncapabilities can be achieved. In a number of embodiments, the depositionhead can allow for the deposition of different materials and/or mixturesthat vary in component concentrations. As can readily be appreciated,the specific deposition mechanism utilized can depend on the specificrequirements of a given application.

The components within the deposition workcell can be configured to movein various ways in order to deposit the optical recording material ontothe substrate. In many embodiments, the deposition head and/or the stageare configured to move across certain axes in order to deposit one ormultiple layers of optical recording material. In some embodiments, thedeposition head is configured to move and deposit material across threedimensions, such as in a three-dimensional Euclidean space, which allowsfor the deposition of multiple layers onto the substrate. In a number ofembodiments, the deposition head is only configured to move in two axesto deposit a single layer. In other embodiments, the stage and,consequently, the substrate are configured to move in three dimensionswhile the deposition head is stationary. As can readily be appreciated,deposition applications can be implemented to deposit material invarious dimensions by configuring the degrees of motion freedom of theprint head(s) and/or stage. The stage and deposition head can beconfigured such that their combination of degrees of motion freedomallows for depositing material in n-dimensional Euclidean space, where nis the desired dimension. For example, in several embodiments, thedeposition head is configured to move back and forth to deposit materialin one axis while the stage moves in a different axis, allowing for thedeposition of material in a two-dimensional Euclidean plane. In a numberof embodiments, the stage is implemented using a conveyor belt. Thesystem can be designed such that the conveyor belt receives thesubstrate from a different workcell, such as the preparation workcell.Once received, the conveyor system can move the substrate along as adeposition head deposits a layer of material onto the substrate. At theend of the conveyor path, the substrate can be delivered into anotherworkcell.

In a number of embodiments, the deposition workcell includes an inkjetprint head configured to deposit optical recording material onto thesubstrate. Conventionally, inkjet printing refers to a printing methodthat deposits a matrix of ink dots to form a desired image. In typicaloperation, an inkjet print head contains a large amount of smallindividual nozzles that can each deposit a dot of material. In additivemanufacturing applications, inkjet printing can be used to createcomplex patterns and structures with high precision due to the size andnumber of nozzles in a typical inkjet print head. Applying theseprinciples to waveguide cell manufacturing applications, inkjet printingcan be used to print a uniform or near-uniform, in terms of thicknessand composition, layer of optical recording material. Depending on theapplication and inkjet print head, one or multiple layers of the opticalrecording material can be printed onto the substrate. Various opticalrecording materials, such as those described in the sections above, canbe used in conjunction with an inkjet print head. In addition to thecapability of printing in different materials, the printing system canbe configured for use with various types of substrates. As can readilybe appreciated, the choice of material to be printed and the substratesused can depend on the specific requirements of a given application. Forinstance, choices in material systems can be selected based on printingstability and accuracy. Other considerations can include but are notlimited to viscosity, surface tension, and density, which can influenceseveral factors such as but not limited to droplet formability and theability to form layers of uniform thickness,

A deposition workcell 300 in accordance with an embodiment of theinvention is conceptually illustrate in FIGS. 3A and 3B. FIG. 3A showsan isometric view of the deposition workcell 300 while FIG. 3B shows atop view of the same deposition workcell 300. As shown, the depositionworkcell 300 is constructed with a frame that can hold optical glassfilters to prevent particulate contamination and environmental lightfrom exposing optical recording materials within the workcell 300. Theworkcell includes chambers 302, 304 for receiving substrates andoutputting waveguide cells. In the illustrative embodiment, the stage isimplemented as a conveyor belt 306 that moves received substrates alongone direction. The deposition workcell 300 further includes an inkjetprinter 308 implemented as a deposition mechanism. The inkjet printer308 is configured to print across a direction different from themovement of the conveyor belt 306, allowing for the deposition of alayer of optical recording material across the planar surface of thesubstrates. Additionally, the deposition workcell 300 implements aroller laminator 310 for laminating the printed layer and two substratesto construct a waveguide cell. The workcell 300 is also implemented as aglovebox with gloves 312 that allow for the manual manipulation of thedevices within the workcell 300 while maintaining a clean environment.

Although FIGS. 3A and 3B depict a specific deposition workcellconfiguration, deposition workcells can be configured in many ways inaccordance with various embodiments of the invention. For example, thelaminator can be implemented in a separate lamination workcell. Inseveral embodiments, automatic system configurations can be implemented.In many embodiments, multiple inkjet print heads are used. In otherembodiments, spraying nozzles are used as the deposition mechanism.

Modulation of Material Composition

High luminance and excellent color fidelity are important factors in ARwaveguide displays. In each case, high uniformity across the FOV can beessential. However, the fundamental optics of waveguides can lead tonon-uniformities due to gaps or overlaps of beams bouncing down thewaveguide. Further non-uniformities may arise from imperfections in thegratings and non-planarity of the waveguide substrates. In SBGs, therecan exist a further issue of polarization rotation by birefringentgratings. The biggest challenge is the fold grating where there aremillions of light paths resulting from multiple intersections of thebeam with the grating fringes. Careful management of grating properties,particularly the refractive index modulation, can be utilized toovercome non-uniformity in accordance with various embodiments of theinvention.

Out of the multitude of possible beam interactions (diffraction or zeroorder transmission), only a subset contributes to the signal presentedat the eye box. By reverse tracing from the eyebox, fold regionscontributing to a given field point can be pinpointed. The precisecorrection to the modulation that is needed to send more into the darkregions of the output illumination can then be calculated. Havingbrought the output illumination uniformity for one color back on target,the procedure can be repeated for other colors. Once the indexmodulation pattern has been established, the design can be exported tothe deposition mechanism, with each target index modulation translatingto a unique deposition setting for each spatial resolution cell on thesubstrate to be coated. In many embodiments, the spatial pattern can beimplemented to 30 micrometers resolution with full repeatability.

FIGS. 4A and 4B conceptually illustrate schematically the use of reverseray tracing to compute a compensated index modulation pattern forcoating in accordance with various embodiments of the invention. Theprocedure can determine the optimum usable area of the fold grating andthe refractive index modulation variation across the fold grating neededto provide uniform illumination at the eye box. FIG. 4A shows amathematical model of a basic waveguide architecture that includes aninput grating 402, a fold grating that is divided up into a calculationmesh 404, and an output grating 406. By tracing rays from points acrossthe eye box through the output grating and through the fold grating, thefold grating cells which contribute to the eyebox illumination for agiven FOV direction can be identified. Reverse beam paths from theoutput grating are indicated by the rays 408-414. By repeating the raytrace for different FOV angles the maximum extent of the fold gratingneeded to fill the eye box can be determined. This ensures that the areaof HPDLC material to be deposited/printed can be kept to a minimum,thereby reducing haze in the finished waveguide part. The procedure canalso identify which cells need to have their index modulation increased(or decreased) in order to maintain illumination uniformity across theeyebox. For example, in the embodiment of FIG. 4A, most of the foldgrating region has a refractive index modulation of 0.03. However,certain calculation cells encircled by 416 (such as cell 418, forexample) and encircled by 420 (such as cell 422, for example) shouldhave index modulations of 0.07, while the calculation cells lying withinthe rectangular zone 424 should have index modulation 0.05. Typically,the map of index modulation values is exported as an AutoCAD DXF(Drawing Interchange Format) file into the processor controlling thedeposition mechanism. FIG. 4B is a plan view 450 of the final waveguidepart 452 onto which is superimposed the index modulation map of theprinted grating layer (corresponding to the model of FIG. 4A) as wouldbe revealed by examining the printed grating under cross polarizers. Thegrating regions include the input 454, output 456, and fold 458gratings. In the illustrative embodiment, the fold grating contains thehigh index modulation regions 460, 462, and 464 corresponding to thecells identified in regions 416, 420, and 424 of FIG. 4A. The gratingregions of FIG. 4B are surrounded by a clear polymer region 466.Although FIGS. 4A and 4B illustrate a specific way of computing acompensated index modulation pattern, any of a variety of techniques canbe utilized to compute such a pattern.

Compared with waveguides utilizing surface relief gratings (“SRGs”), SBGwaveguides implementing manufacturing techniques in accordance withvarious embodiments of the invention can allow for the grating designparameters that impact efficiency and uniformity, such as refractiveindex modulation and grating thickness, to be adjusted dynamicallyduring the deposition process. As such, there is no need for a newmaster for the grating recording process. With SRGs where modulation iscontrolled by etch depth, such schemes would not be practical as eachvariation of the grating would entail repeating the complex andexpensive tooling process. Additionally, achieving the required etchdepth precision and resist imaging complexity can be very difficult.FIGS. 5A and 5B conceptually illustrate the fundamental structuraldifferences between SBGs and SRGs. FIG. 5A shows a cross-sectional view500 of a portion of an SRG. In the illustrative embodiment, the gratingincludes a substrate 502 supporting slanted surface relief elements 504separated by air gaps 506. Typically, the surface relief elements andsubstrate are formed from a common material. The grating pitch isindicated by the symbol p and the grating depth by symbol h. FIG. 5Bshows a cross-sectional view 550 of an SBG. In contrast to an SRG, theSBG includes alternating slanted Bragg fringes formed from low indexmonomer-rich fringes such as 552 and higher index LC-rich fringes suchas 554. The index difference is characterized by the refractive indexmodulation δn, which plays an equivalent role in determining gratingdiffraction efficiency to the grating depth in a SRG. The variation ofindex modulation is represented by the superimposed plot 556 of indexmodulation versus distance z along the grating. In some embodiments, theindex modulation has a sinusoidal profile as shown in FIG. 5B. Inembodiments in which the SBG is formed in a uniform modulation HPLDC,the index modulation profile can include near-rectangular LC-rich andpolymer-rich regions.

Deposition processes in accordance with various embodiments of theinvention can provide for the adjustment of grating design parameters bycontrolling the type of material that is to be deposited. Similar tomulti-material additive manufacturing techniques, various embodiments ofthe invention can be configured to deposit different materials, ordifferent material compositions, in different areas on the substrate. Inmany embodiments, a layer of optical recording material can be depositedwith different materials in different areas. For example, depositionprocesses can be configured to deposit HPDLC material onto an area of asubstrate that is meant to be a grating region and to deposit monomeronto an area of the substrate that is meant to be a nongrating region.In several embodiments, the deposition process is configured to deposita layer of optical recording material that varies spatially in componentcomposition, allowing for the modulation of various aspects of thedeposited material. Modulation schemes and deposition processes fordifferent types of materials and mixtures are discussed below in furtherdetail.

The choice in material printed in a specific area can depend on theoptical element that will later be recorded in that area. For example,in some embodiments, the deposition head is configured to deposit alayer of optical recording material for a waveguide cell intended to berecorded with three different gratings. The layer can be deposited suchthat the materials printed in each of the areas designated for the threegratings are all different from one another. FIG. 6 conceptuallyillustrates a waveguide cell 600 with marked areas intended to berecorded with various gratings in accordance with an embodiment of theinvention. As shown, areas for an input grating 602, a fold grating 604,and an output grating 606 are outlined. Such areas can each be composedof a different material or different mixture composition depending onthe given application. In a number of embodiments, different materialscan be deposited to produce different diffraction efficiencies among therecorded gratings. In the illustrative embodiment, the waveguide cell isin a curvilinear shape, which, along with the positions, sizes, andshapes of the gratings, is designed to be a waveguide for near-eyeapplications.

Deposition of material with different compositions can be implemented inseveral different ways. In many embodiments, more than one depositionhead can be utilized to deposit different materials and mixtures. Eachdeposition head can be coupled to a different material/mixturereservoir. Such implementations can be used for a variety ofapplications. For example, different materials can be deposited forgrating and nongrating areas of a waveguide cell. In some embodiments,HPDLC material is deposited onto the grating regions while only monomeris deposited onto the nongrating regions. In several embodiments, thedeposition mechanism can be configured to deposit mixtures withdifferent component compositions.

In some embodiments, spraying nozzles can be implemented to depositmultiple types of materials onto a single substrate. In waveguideapplications, the spraying nozzles can be used to deposit differentmaterials for grating and non-grating areas of the waveguide. FIGS. 7Aand 7B conceptually illustrate operation of a deposition mechanismutilizing a spray module in accordance with an embodiment of theinvention. As shown, the apparatus 700 includes a coating module 702that includes a first spray module 704 connected via a pipe 706 to afirst reservoir 708 containing a first mixture of a first material and asecond spray module 710 connected via a pipe 712 to a second reservoir714 containing a second mixture of a second material. In theillustrative embodiment, the first material includes at least a liquidcrystal and a monomer while the second material includes only a monomer.Such a configuration allows for the deposition of a layer of opticalrecording material with defined grating and non-grating areas. As canreadily be appreciated, any configurations of different mixtures can beutilized as appropriate depending on the specific application.

In FIGS. 7A and 7B, the first and second spray modules provide jets ofliquid droplets over a controllable divergence angle as represented by716, 718. The apparatus further includes a support for a transparentsubstrate 720 having predefined regions for supporting gratings asillustrated by the shaded regions 722-726, regions of gratings that donot transmit light into the eyebox as indicated by 728, 730, and regionssurrounding the gratings indicated by 732. In some embodiments, theregions 728, 730 are identified by a reverse ray trace of the waveguidefrom the eyebox. During operation, the regions for supporting gratingsproviding diffracted light that enters the eye box are coated with thefirst mixture. The regions 728, 730 are coated with the second mixture.The apparatus further includes a positioning apparatus 734 connected tothe coating apparatus by a control link 736 for traversing the coatingapparatus across the substrate. The apparatus further includes aswitching mechanism for activating the first spray module anddeactivating the second spray module when the coating apparatus ispositioned over a substrate region for supporting a grating and fordeactivating the first spray module and activating the second spraymodule when the coating apparatus is positioned over a substrate regionthat does not support a grating.

Two operational states of the apparatus are conceptually illustrated inFIGS. 8A and 8B, which show a detail of the substrate. As shown in FIG.8A, when the coating apparatus is over a nongrating-supporting region800 (located in the upper region of the strip bounded by the edges 802,804), the second spray module is activated, and the first spray moduleis deactivated so that a layer of monomer 806 is sprayed onto thesubstrate. As shown in FIG. 8B, when the coating apparatus is over asubstantially grating-supporting region 808 (located in the lower regionof the strip bounded by the edges 802, 804), the second spray module isdeactivated, and the first spray module is activated so that a layer ofliquid crystal and monomer mixture 810 is sprayed onto the substrate.

Although FIGS. 7A-8B illustrate specific applications and configurationsof spraying mechanisms, spraying mechanisms and deposition mechanisms ingeneral can be configured and utilized for a variety of applications. Inmany embodiments, the spraying mechanism is configured for printinggratings in which at least one of the material composition,birefringence, and thickness can be controlled using a coating apparatushaving at least two selectable spray heads. In some embodiments, thedeposition workcell provides an apparatus for depositing gratingrecording material optimized for the control of laser banding. Inseveral embodiments, the deposition workcell provides an apparatus fordepositing grating recording material optimized for the control ofpolarization non-uniformity. In some embodiments, the depositionworkcell provides an apparatus for depositing grating recording materialoptimized for the control of polarization non-uniformity in associationwith an alignment control layer. In a number of embodiments, thedeposition workcell can be configured for the deposition of additionallayers such as beam splitting coatings and environmental protectionlayers. Additionally, although FIGS. 7A-8B discuss the capabilities ofspraying nozzles, these capabilities can be implemented in otherdeposition mechanisms. For example, inkjet print heads can also beimplemented to print different materials in grating and nongratingregions of the substrate.

FIG. 9 is a flow chart conceptually illustrating a method of fabricatinga holographic grating using a selective coating process in accordancewith an embodiment of the invention. Referring to FIG. 9, the method 900includes providing (902) a transparent substrate for coating. A gratingsupporting and non-grating-supporting regions of the substrate can bedefined (904). Depending on the specific application, gratings ofvarious sizes and shapes can be defined. In some embodiments, a gratingregion supports an input, a fold, or an output grating. In manyembodiments, the substrate has regions defined for gratings made of acombination of the aforementioned types of gratings. A first mixture forcoating containing a liquid crystal and monomer and a second mixture forcoating containing a monomer can be provided (906). A first spray headcan be provided (908) for coating the first mixture onto the substrate.A second spray head can be provided (910) for coating the secondmixture. The first and second spray heads integrated together can beconsidered a coating apparatus. The coating apparatus can be set (912)to its starting position (k=1). The coating apparatus can be moved (914)to the current position over the substrate. A decision can be made (916)on whether the current coating apparatus is positioned over a gratingsupporting region or a non-grating-supporting region. If the coatingapparatus is over a grating region, the first spray head can beactivated and the second spray head can be deactivated (918). If thecoating module is over a grating-supporting region, the first spray headcan be deactivated and the second spray head can be activated (920). Adecision can be made (922) regarding the coating status. If allspecified regions have been coated, the process can be terminated (924).If the specified regions have not all been coated, the next region(increment k) to be coated can be selected (926) and the depositionsteps can be repeated.

Although FIG. 9 illustrates a specific method for depositing differentmaterials over a substrate, the deposition mechanism can be configuredto produce a film of material having characteristics that can varyspatially and across regions. FIG. 10 conceptually illustrates adeposition head for providing predefined grating characteristics withingrating regions in accordance with an embodiment of the invention.Referring to FIG. 10, the deposition head 1000 includes a first spraymodule 1002 fed via pipe 1004 from a reservoir 1006 containing a mixtureof at least one of a liquid crystal and a monomer, which is dispersedinto the spray jet 1008 by the spray module 1002 for coating atransparent substrate. The substrate has predefined regions forsupporting gratings. There is also provided an X-Y displacementcontroller 1010 for traversing the spray module across the substrate anda means for controlling the spray characteristics from the module overeach grating region to deposit a film that provides a predefined gratingcharacteristic within the grating region following holographic exposure.The holographic exposure may be carried out using any currentholographic process, include any of the processes disclosed in thereference documents. In the illustrative embodiment, the deposition head1000 further includes a mixture controller 1012 for controlling one ormore of the temperature, dilution and relative concentrations ofchemical components of the mixture. The deposition head 1000 can alsoinclude a spray controller 1014 for controlling one or more of the sprayangle relative to the substrate, the spray divergence angle, and thedurations of the spray on and off states. In several embodiments, thepredefined grating characteristic includes one or more of refractiveindex modulation, refractive index, birefringence, liquid crystaldirector alignment, and grating layer thickness. As can readily beappreciated, deposition heads can be implemented and configured in manydifferent ways. In many embodiments, any combination and subset of theX-Y displacement controller, mixture controller, and spray controllercan be utilized. In some embodiments, additional controllers areutilized to configure the spraying mechanism and the material deposited.

FIG. 11 conceptually illustrates operation of a deposition head fordepositing material having regions with predefined gratingcharacteristics in accordance with an embodiment of the invention. Asdiscussed above, the deposition head can be configured to depositmaterial having a spatial variation across the grating region of one ormore of refractive index modulation, refractive index, birefringence,liquid crystal director alignment and grating layer thickness. As shownin FIG. 11, the spray module 1100 follows a spraying path 1102 acrossthe substrate 1104. The spray can be dynamically controlled duringtransit along the path 1102 to vary the predefined gratingcharacteristics in areas of the predefined grating regions such as 1106,1108, for example. In some embodiments, the deposition mechanismprovides, after exposure, a grating with a spatially varying diffractionefficiency. For example, referring again to FIG. 11, the coating areas1106, 1108 (after holographic exposure) exhibit diffraction efficiency(DE) versus angle (U) characteristics represented by the curves 1110,1112 respectively.

FIG. 12 conceptually illustrates a deposition mechanism for depositingtwo grating layers in accordance with an embodiment of the invention. Asshown, the system 1200 is similar to that of FIG. 11 but furtherincludes a second spray module 1202 providing a jet 1204 for coating thesecond grating layer 1206. In many embodiments, the grating layers arecoated using different mixture compositions. In some embodiments,similar to the one of FIG. 7A, the system includes a first spray moduleconnected to a first reservoir containing a first mixture that includesat least one of a first liquid crystal and a first monomer and a secondspray module connected to a second reservoir containing a second mixturethat includes at least one of a second liquid crystal and a secondmonomer.

FIG. 13 conceptually illustrates a system for depositing a grating layerof material and for holographically exposing the layer using recordingbeams with on and off states synchronized with the coating module. Asshown, the system 1300 includes a coating apparatus similar to that ofFIG. 12 following a spraying path 1302 across the substrate 1304providing predefined grating regions 1306, 1308. While the coatingprocess is taking place, a holographic exposure apparatus 1310, whichprovides a recording beam 1312, can expose coated predefined gratingregions 1314. In many embodiments, the holographic exposure apparatus isbased on a master grating which contact copies the required grating intothe predefined grating region.

FIG. 14 is a flow chart conceptually illustrating a method of depositinga film of material with regions having predefined gratingcharacteristics in accordance with an embodiment of the invention. Asshown, the method 1400 includes providing (1402) a transparent substratefor coating. A grating supporting and non-grating-supporting regions ofthe substrate can be defined (1404). A mixture containing a liquidcrystal and monomer can be provided (1406). In several embodiments, thematerial utilized includes one or more of a photoinitiator,nano-particles, low-functionality monomers, additives for reducingswitching voltage, additives for reducing switching time, additives forincreasing refractive index modulation and additives for reducing haze.A spray module for coating the mixture onto the substrate can beprovided (1408). The spray module can be set (1410) to its startingposition (k=1). The spray module can be moved (1412) to the currentposition over the substrate. A decision can be made (1414) on whetherthe current coating apparatus is positioned over a grating supportingregion or a non-grating-supporting region. If the coating apparatus isover a grating region, the spray module can be activated (1416) toprovide a spray characteristic for achieving a predefined gratingcharacteristic within the grating region. The grating region can becoated (1418). A decision can be made (1420) regarding the coatingstatus. If all specified regions have been coated, the process can beterminated (1422). If all specified regions have not been coated, thenext region to be coated can be selected (1424) and the deposition stepscan be repeated with k incremented.

Although FIGS. 10-14 illustrate specific implementations and methods ofdepositing material with regions having predefined gratingcharacteristics, any of a variety of configurations can be implemented.For example, in many embodiments, multiple spray modules or depositionheads are utilized. Various predefined grating characteristics can becontrolled and/or modulated depending on the specific application.Modulation of material composition utilizing more than one depositionhead is discussed below in further detail.

As discussed above, deposition processes can be configured to depositoptical recording material that varies spatially in componentcomposition. Modulation of material composition can be implemented inmany different ways. In a number of embodiments, an inkjet print headcan be configured to modulate material composition by utilizing thevarious inkjet nozzles within the print head. By altering thecomposition on a “dot-by-dot” basis, the layer of optical recordingmaterial can be deposited such that it has a varying composition acrossthe planar surface of the layer. Such a system can be implemented usinga variety of apparatuses including but not limited to inkjet printheads. Similar to how color systems use a palette of only a few colorsto produce a spectrum of millions of discrete color values, such as theCMYK system in printers or the additive RGB system in displayapplications, inkjet print heads in accordance with various embodimentsof the invention can be configured to print optical recording materialswith varying compositions using only a few reservoirs of differentmaterials. Different types of inkjet print heads can have differentprecision levels and can print with different resolutions. In manyembodiments, a 300 DPI (“dots per inch”) inkjet print head is utilized.Depending on the precision level, discretization of varying compositionsof a given number of materials can be determined across a given area.For example, given two types of materials to be printed and an inkjetprint head with a precision level of 300 DPI, there are 90,001 possiblediscrete values of composition ratios of the two types of materialsacross a square inch for a given volume of printed material if each dotlocation can contain either one of the two types of materials. In someembodiments, each dot location can contain either one of the two typesof materials or both materials. In several embodiments, more than oneinkjet print head is configured to print a layer of optical recordingmaterial with a spatially varying composition. Although the printed dotsfor a two-material application are essentially a binary system, inpractical applications, averaging the printed dots across an area canallow for discretization of a sliding scale of ratios of the twomaterials to be printed.

FIG. 15 conceptually illustrates an inkjet printing modulation scheme inaccordance with an embodiment of the invention. As shown, eighteendiscrete unit-squares are each capable of being printed with a varyingratios of two different types of materials. In the illustratedembodiment, the inkjet print head is capable of printing sixty-four dotswithin each of the eighteen unit squares. Each dot can be printed witheither one of two types of material. A close up 1500 of unit square 1502shows that all sixty-four dot locations within the unit square isprinted with the first material. Similarly, a close up 1504 of unitsquare 1506 is printed completely with the second material. Unit square1508 shows an intermediate composition where thirty out of thesixty-four dot locations are printed with the first material while theremaining dot locations are printed with the second material. As such,unit square 1508, as a whole, contains an intermediate level ofconcentrations from both materials. Utilizing this modulation scheme,any pattern of varying material characteristics can be achieved.

The amount of discrete levels of possible concentrations/ratios across aunit square is given by how many dot locations can be printed within theunit square. In the illustrative embodiment, sixty-four discrete dotscan be printed within the unit square, which thus results in each unitsquare having a possibility of sixty-five different concentrationcombinations, ranging from 100% of the first material to 100% of thesecond material. Although FIG. 15 discusses the areas in terms of a unitsquare, the concepts are applicable to real units and can be determinedby the precision level of the inkjet print head. Although specificexamples of modulating the material composition of the printed layer arediscussed, it can readily be appreciated that the concept of modulatingmaterial composition using inkjet print head can be expanded to use morethan two different material reservoirs and can vary in precision levels,which largely depends on the type of print head used.

Varying the composition of the material printed can be advantageous forseveral reasons. For example, in many embodiments, varying thecomposition of the material during deposition can allow for a waveguidewith gratings that have varying diffraction efficiencies acrossdifferent areas of the gratings. In embodiments utilizing HPDLCmixtures, this can be achieved by modulating the relative concentrationof liquid crystals in the HPDLC mixture during the printing process,which creates compositions that can produce gratings with varyingdiffraction efficiencies when exposed. In several embodiments, a firstHPDLC mixture with a certain concentration of liquid crystals and asecond HPDLC mixture that is liquid crystal-free are used as theprinting palette in an inkjet print head for modulating the diffractionefficiencies of gratings that can be formed in the printed material. Insuch embodiments, discretization can be determined based on theprecision of the inkjet print head. For example, if a 150 DPI inkjetprint head is utilized, each square inch can be printed with 22,501discrete levels of liquid crystal concentration. A discrete level can begiven by the concentration/ratio of the materials printed across acertain area. In this example, the discrete levels range from no liquidcrystal to the maximum concentration of liquid crystals in the firstPDLC mixture.

The ability to vary the diffraction efficiency across a waveguide can beused for various purposes. Waveguides are typically designed such thatlight can be reflected many times between the two planar surfaces of awaveguide. These multiple reflections can allow for a light path tointeract with a grating multiple times. In many embodiments, a waveguidecell can be printed with varying compositions such that the gratingsformed from the optical recording material layer have varyingdiffraction efficiencies to compensate for the loss of light duringinteractions with the gratings to allow for a uniform output intensity.For example, in some waveguide applications, an output grating isconfigured to provide exit pupil expansion in one direction while alsocoupling light out of the waveguide. The output grating can be designedsuch that when light within the waveguide interact with the grating,only a percentage of the light is refracted out of the waveguide. Theremaining portion continues in the same light path, which remains withinTIR and continues to be reflected within the waveguide. Upon a secondinteraction with the same output grating again, another portion of lightis refracted out of the waveguide. During each refraction, the amount oflight still traveling within the waveguide decreases by the amountrefracted out of the waveguide. As such, the portions refracted at eachinteraction gradually decreases in terms of total intensity. By varyingthe diffraction efficiencies of the grating such that it increases withpropagation distance, the decrease in output intensity along eachinteraction can be compensated, allowing for a uniform output intensity.

Varying the diffraction efficiency can also be used to compensate forother attenuation of light within a waveguide. All objects have a degreeof reflection and absorption. Light trapped in TIR within a waveguideare continually reflected between the two surfaces of the waveguide.Depending on the material that makes up the surfaces, portions of lightcan be absorbed by the material during each interaction. In many cases,this attenuation is small, but can be substantial across a large areawhere many reflections occur. In many embodiments, a waveguide cell canbe printed with varying compositions such that the gratings formed fromthe optical recording material layer have varying diffractionefficiencies to compensate for the absorption of light from thesubstrates. Depending on the substrates, certain wavelengths can be moreprone to absorption by the substrates. In a multi-layer waveguidedesign, each layer can be designed to couple in a certain range ofwavelengths of light. Accordingly, the light coupled by these individuallayers can be absorbed in different amounts by the substrates of thelayers. For example, in a number of embodiments, the waveguide is madeof a 3-layer stack to implement a color display, where each layer isdesigned for one of Red, Green, and Blue. In such embodiments, gratingswithin each of the waveguide layers can be formed to have varyingdiffraction efficiencies to perform color balance optimization bycompensating for color imbalance due to loss of transmission of certainwavelengths of light.

In addition to varying the liquid crystal concentration within thematerial in order to vary the diffraction efficiency, another techniqueincludes varying the thickness of the waveguide cell. This can beaccomplished through the use of beads. In many embodiments, beads aredispersed throughout the optical recording material for structuralsupport during the construction of the waveguide cell. In someembodiments, different sizes of beads are dispersed throughout theoptical recording material. The beads can be dispersed in ascendingorder of sizes across one direction of the layer of optical recordingmaterial. When the waveguide cell is constructed through lamination, thesubstrates sandwich the optical recording material and, with structuralsupport from the varying sizes of beads, create a wedge shaped layer ofoptical recording material. Beads of varying sizes can be dispersedsimilar to the modulation process described above. Additionally,modulating bead sizes can be combined with modulation of materialcompositions. In several embodiments, reservoirs of HPDLC materials eachsuspended with beads of different sizes are used to print a layer ofHPDLC material with beads of varying sizes strategically dispersed toform a wedge shaped waveguide cell. In a number of embodiments, beadsize modulation is combined with material composition modulation byproviding an amount of reservoirs equal to the product of the number ofdifferent sizes of beads and the number of different materials used. Forexample, in one embodiment, the inkjet print head is configured to printvarying concentrations of liquid crystal with two different bead sizes.In such an embodiment, four reservoirs can be prepared: a liquidcrystal-free mixture-suspension with beads of a first size, a liquidcrystal-free mixture-suspension with beads of a second size, a liquidcrystal-rich mixture-suspension with beads of a first size, and a liquidcrystal-rich mixture-suspension with beads of a second size.

Lamination Workcell

In many embodiments, the workcell cluster includes a lamination workcellfor laminating the waveguide cell. After the deposition of opticalrecording material onto a substrate, a second substrate can be placedonto the optical recording material, creating a three-layer composite.Oftentimes, the second substrate will be made of the same material andin the same dimensions as the first substrate. In many embodiments, thedeposition workcell is configured to place the second substrate onto theoptical recording material. In other embodiments, the laminationworkcell is configured to place the second substrate onto the opticalrecording material. The second substrate can be placed manually orthrough the use of mechanical arms and/or suction mechanisms. Once thesecond substrate is placed, the three-layer composite may be toounstable to handle manually and, thus, in many embodiments, a laminatoris implemented to compact the composite.

The three-layer composite can be laminated in various ways. In manyembodiments, a press is implemented to provide downward pressure ontothe composite. In other embodiments, the lamination workcell isconfigured to feed the composite through a roller laminator. Thecompacted composite and adhesion properties of the optical recordingmaterial can result in a waveguide cell with enough stability to behandled manually. In some embodiments, the layer of optical recordingmaterial includes beads. Consequently, these relatively incompressiblebeads can define the height of the layer of optical recording materialwithin the compacted composite. As discussed in the sections above,differently sized beads can be placed throughout the optical recordingmaterial. Upon lamination, the sizes of the beads can each determine thelocal thickness of the waveguide cell. By varying the sizes of thebeads, a wedge shaped waveguide cell can be constructed. As can readilybe appreciated, the lamination of the substrates-optical recordingmaterial layer composite can be achieved using lamination workcells thatcan be configured and implemented in many different ways. In severalembodiments, the lamination workcell is a modular workcell within theworkcell cluster. In other embodiments, the lamination workcell issimply a laminator implemented within the deposition workcell, such asthe one shown in FIGS. 3A and 3B.

Although specific systems and methods for manufacturing waveguide cellsare discussed above, many different configurations can be implemented inaccordance with many different embodiments of the invention. It istherefore to be understood that the present invention can be practicedin ways other than specifically described, without departing from thescope and spirit of the present invention. Thus, embodiments of thepresent invention should be considered in all respects as illustrativeand not restrictive. Accordingly, the scope of the invention should bedetermined not by the embodiments illustrated, but by the appendedclaims and their equivalents.

What is claimed is:
 1. A method for manufacturing waveguide cells, themethod comprising; providing a first substrate; determining a predefinedgrating characteristic; and depositing a layer of optical recordingmaterial onto the first substrate using at least one deposition head,wherein the optical recording material deposited over the grating regionis formulated to achieve the predefined grating characteristic.
 2. Themethod of claim 1, further comprising: providing a second substrate;placing the second substrate onto the deposited layer of opticalrecording material; and laminating the first substrate, the layer ofoptical recording material, and the second substrate.
 3. The method ofclaim 1, wherein depositing the layer of optical recording materialcomprises: providing a first mixture of optical recording material;providing a second mixture of optical recording material; and depositingthe first and second mixtures of optical recording material onto thefirst substrate in a predetermined pattern using the at least onedeposition head.
 4. The method of claim 3, wherein: the first mixture ofoptical recording material comprises a first bead; and the secondmixture of optical recording material comprises a second bead that is adifferent size from the first bead.
 5. The method of claim 3, whereinthe first mixture of optical recording material has a differentpercentage by weight of liquid crystals than the second mixture ofoptical recording material.
 6. The method of claim 3, further comprisingdefining a grating region and a nongrating region on the firstsubstrate, wherein: the first mixture of optical recording materialcomprises a liquid crystal and a monomer; the second mixture of opticalrecording material comprises a monomer; and depositing the first andsecond mixtures of optical recording material onto the first substratein the predetermined pattern comprises: depositing the first mixture ofoptical recording material over the grating region; and depositing thesecond mixture of optical recording material over the nongrating region.7. The method of claim 3, wherein the first mixture of optical recordingmaterial is a polymer dispersed liquid crystal mixture comprising: amonomer; a liquid crystal; a photoinitiator dye; and a coinitiator. 8.The method of claim 7, wherein the polymer dispersed liquid crystalmixture comprises an additive selected from the group consisting of: aphotoinitiator, nano particles, low-functionality monomers, additivesfor reducing switching voltage, additives for reducing switching time,additives for increasing refractive index modulation, and additives forreducing haze.
 9. The method of claim 1, wherein the at least onedeposition head comprises at least one inkjet print head.
 10. The methodof claim 8, wherein depositing the layer of optical recording materialcomprises: providing a first mixture of optical recording material;providing a second mixture of optical recording material; printing afirst dot of the first mixture of optical recording material using theat least one inkjet print head; and printing a second dot of the secondmixture of optical recording material adjacent to the first dot usingthe at least one inkjet print head.
 11. The method of claim 9, wherein:the at least one inkjet print head comprises a first inkjet print headand a second inkjet print head; and depositing the layer of opticalrecording material comprises: providing a first mixture of opticalrecording material; providing a second mixture of optical recordingmaterial; printing the first mixture of optical recording material ontothe first substrate using the first inkjet print head; and printing thesecond mixture of optical recording material onto the first substrateusing the second inkjet print head.
 12. The method of claim 1, whereinthe predefined grating characteristic comprises a characteristicselected from the group consisting of: refractive index modulation,refractive index, birefringence, liquid crystal director alignment, andgrating layer thickness.
 13. The method of claim 1, wherein thepredefined grating characteristic comprises a spatial variation of acharacteristic selected from the group consisting of: refractive indexmodulation, refractive index, birefringence, liquid crystal directoralignment, and grating layer thickness.
 14. The method of claim 1,wherein the predefined grating characteristic results in a grating afterexposure, the grating having a spatially varying diffraction efficiency.15. A system for fabricating a grating, the system comprising: at leastone deposition head connected to at least one reservoir containing atleast one mixture of optical recording material; a first substratehaving at least one predefined region for supporting gratings; apositioning element capable of positioning the at least one depositionhead across the first substrate, wherein: the at least one depositionhead is configured to deposit the at least one mixture of opticalrecording material onto the first substrate using the positioningelement; and the deposited material provides a predefined gratingcharacteristic within the at least one predefined grating region afterholographic exposure.
 16. The system of claim 15, wherein the at leastone deposition head is connected to a first reservoir containing a firstmixture of optical recording material and a second reservoir containinga second mixture of optical recording material.
 17. The system of claim16, wherein the first mixture of optical recording material comprises aliquid crystal and a monomer; and the second mixture of opticalrecording material comprises a monomer; wherein the at least onedeposition head is configured to deposit the first mixture of opticalrecording material onto the at least one predefined grating region. 18.The system of claim 15, wherein the at least one deposition headcomprises at least one inkjet print head.
 19. The system of claim 15,wherein the predefined grating characteristic comprises a characteristicselected from the group consisting of: refractive index modulation,refractive index, birefringence, liquid crystal director alignment, andgrating layer thickness.
 20. The system of claim 15, wherein thepredefined grating characteristic results in a grating after exposure,the grating having a spatially varying diffraction efficiency.